As electric appliances such as cellular phone and the like have become smaller and lighter and have come to possess diversified functions, lithium ion secondary battery used therein is required to have a higher capacity.
The current high capacity of lithium ion secondary battery has been achieved partly by the improvement in structure and material of lithium ion secondary battery and the use of positive electrode material of high capacity. However, the biggest contribution to the high capacity of lithium ion secondary battery can be said to be the increase in bulk density (electrode density), realized for the negative electrode produced using graphite particles.
However, when the electrode density of negative electrode is increased to 1.7 g/ml or more, problems observed seldom at low electrode densities appear markedly. They are specifically reduction in diffusion of electrolytic solution into electrode, reduction in high rate charge-discharge characteristic, deterioration in cycle life characteristic, etc. Therefore, it is difficult to allow lithium ion secondary battery to have an even higher capacity by allowing a negative electrode produced using graphite to have a higher density.
Hence, the present inventor made an investigation on the negative electrode produced using the carbon-graphite composite particles obtained by applying a chemical vapor deposition (CVD) treatment on the surfaces of graphite particles (see, for example, Patent Literature 1). The negative electrode produced using such carbon-graphite composite particles shows good diffusion of electrolytic solution into electrode and high rate charge-discharge characteristic of lithium ion even when the negative electrode has been molded so as to have an electrode density of 1.7 g/ml or more. On the other hand, the negative electrode has problems such as swelling and peeling of electrode, deterioration in cycle life characteristic and the like. Therefore, with the carbon-graphite composite particles, when they are used as a negative electrode material, it is necessary to mix them with a material of good cycle life characteristic or add thereto a conductive material.
Mixing or addition of such an auxiliary material or conductive aid incurs the impairment by auxiliary material, of the good diffusion of electrolytic solution into electrode and high rate charge-discharge characteristic which are characteristic of carbon-graphite composite particles.
Patent Literature 1: JP-A-2001-213615 (Claims)